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A 23-year-old New York father who police say pulled a knife on another man and threatened to kill him in front of his family on board a subway train after the latter criticised him for striking his own son broke down in tears before a judge on Wednesday.

Edgar Rodriguez, of Roosevelt Island, was arrested on Tuesday on charges of endangering the welfare of a child, menacing and criminal possession of a weapon, the Daily Mail reported.

Rodriguez was caught on video boasting that he was 'tough' after allegedly hitting his toddler son. Photo / Supplied

The Good Samaritan's well-meaning comment prompted the suspect to unleash a foul-mouthed tirade at him.

The portion of the recording released by the NYPD does not show the slap but starts with the suspect yelling at the other rider, "I'm tough by myself, bro."

Attn E train riders: This man is wanted for menacing and we need your help identifying him - Please share this video of the June 24th incident at West 4th St station- Call #800577TIPS with information @NYPD6Pctpic.twitter.com/oJvamOhe5X